1. Field of the Invention
The present invention relates to a memory device and, more particularly, to a semiconductor memory device including memory cells for storing n-valued information.
2. Description of the Related Art
With the progressive advancement in semiconductor technology, the capacity of the semiconductor memory devices is becoming increasingly large. However, the capacity increase has been achieved so far chiefly relying on miniaturizing technology of the memory cells for storing binary information. Therefore, it has also become apparent that the conventional method for capacity increase has its limitations. Meanwhile, on the user side, it is becoming necessary more and more to process a large volume of data or the like at high speed, and therefore, there is no end to requirement for semiconductor memory devices with larger capacity.
Conventionally, the memory capacity is sometimes increased substantially by data compression, that is, compressed data is stored in a smaller area of a semiconductor memory device. More specifically, in the case of text data, data is compressed by Huffman method, and in the case of image data in facsimile, for example, data is compressed by the MH (Modified Huffman) method. Data thus compressed is stored.
On the other hand, the Huffman method compresses data by assigning codes having short bit length to characters which occur with high frequency. Also in the MH method, short codes are assigned to patterns which occur with high frequency. Therefore, those methods are used for compression and restoration of data in units of data blocks with specified block length, so that those methods can only be used for image data accessed sequentially or text data for batch processing. Thus, those methods had no practical use for data that is accessed randomly in byte units, for example, on the semiconductor memory device.